This invention relates to a pelvimetry teaching phantom to be used to teach students and others how to recognize pelvic types and to perform pelvimetric mensurations, or to demonstrate pelvimetry to patients.
The progress and final outcome of labor are influenced by four factors called the "passage", the "powers", the "passenger" and the "placenta". Abnormalities of any of these components, either singly or in combination, may result in dystocia. The first and last are not subject to change by therapeutic manipulation during delivery.
Pelvimetry is defined as pelvic mensuration; it is the clinical measurement of the bony pelvis, and may be either external or internal, or both, and may be performed manually or by imaging. Pelvimetry is a means of determining whether or not the pelvic architecture and dimensions are sufficient to enable a normal-sized fetus to pass through the bony pelvis. External pelvimetry is the measurement of the external pelvic diameters: external conjugate, interspinous, intercristal, intertrochanteric, transverse of the outlet, and oblique diameters. Internal pelvimetry, performed by an internal pelvic examination, is the measurement of the internal pelvic diameters. The pelvic outlet is characterized by the anteroposterior, anterior sagittal, bispinous, and posterior sagittal diameters; and the pelvic inlet by the diagonal conjugate, obstetric conjugate, and true conjugate diameters.
In evaluating the course and conduct of labor, a thorough knowledge of the pelvic landmarks and their spatial relationships is mandatory, including the pelvic inlet, the midpelvis and the pelvic outlet. Assessment of the pelvic inlet includes evaluation of the true, diagonal and obstetric conjugates and the plane of inlet; of the midpelvis symphysis, ischial spines, sacrum (S3-S4), anteroposterior and interspinous diameters; of the pelvic outlet subpubic arch, ischial tuberosities, tip of the sacrum (not the coccyx), intertuberous distance, anteroposterior and posterior sagittal diameters, degree of pelvic wall convergence, and curve of Carus. Table I below sets forth certain pelvimetric characteristics of the pure pelvic types:
TABLE I __________________________________________________________________________ Pelvimetric PELVIC TYPE Character Gynecoid Android Anthropoid Platypelloid __________________________________________________________________________ Widest transverse 12 cm 12 cm less than 12 cm 12 cm diameter of inlet Anteroposterior 11 cm 11 cm greater than 12 cm 10 cm diameter of inlet Side walls Straight Convergent Narrow Wide Forepelvis Wide Narrow Divergent Straight Sacrosciatic notch Medium Narrow Backward Forward Inclination of Medium Forward Wide Narrow sacrum (lower 1/3) Ischial spines Not prominent Prominent Not Prominent Not Prominent Suprapubic arch Wide Narrow Medium Wide Transverse diameter 10 cm less 10 cm 10 cm 10 cm of outlet Bone structure Medium Heavy Medium Medium __________________________________________________________________________ (Benson, RC: Current Obstetric and Gynecological Diagnosis and Treatment, Fifth Edition. Los Altos, CA, Lange Medical Publications, 1984)
Variations in pelvic architecture must be carefully evaluated by the attending medical personnel (obstetrician, OB nurse, midwife, accoucheur) since the progress of delivery is directly determined by the sequence of attitudes and positions the fetus must assume in its passage through the birth canal. For this reason, an accurate assessment of the pelvic architecture and diameters is an important part of obstetric care.
Of essential concern is the true (rather than the false) pelvis, which includes the inlet, the midpelvis, and the outlet. Modern concepts of obstetric pelvic types and their influence on the conduct of labor are based for the most part on the classic work of Caldwell and Moldy in the 1930s. The four basic pelvic types identified by them and generally adopted throughout the world are the gynecoid, android, anthropoid and platypelloid pelvic configurations. Significantly, most pelves are "mixed" types, the anterior segment resembling one type and the posterior segment another.
Four pure types of female pelvis exist, described as gynecoid, anthropoid, android and platypelloid (flat). Additionally, 10 mixed pelvic types exist, and are described as follows, with the first term designating the posterior segment, the second term designating the anterior segment: anthropoid-gynecoid, gynecoid-anthropoid, anthropoid-android, gynecoid-android, gynecoid-flat, android-gynecoid, android-anthropoid, android-flat, flat-android, flat-gynecoid. It is physically impossible to combine anthropoid-flat or flat-anthropoid; these combinations do not exist, with the exception that a platypelloid rachitic pelvic may show anthropoid characteristics.
On occasion, one will encounter an asymmetric pelvis that does not fit the above classification.
Pelvic contractures must be considered. The most common cause of inlet contraction is rickets, affecting 5% of black women in the USA. Childhood debilitating diseases may lead to poor pelvic development, thus a generally contracted pelvis. Trauma and fracture to the bony pelvis and known or suspected congenital deformities are also causes of contracture and dystocia.
In relating pelvic type to the outcome of labor, while the probability of serious arrest in the case of pure gynecoid pelvis is 5.6%, this probability increases dramatically to: gynecoid-android=25.0%, gynecoid-anthropoid=24.0%, gynecoid-flat=12.0%, android (pure)=25.0%, android-gynecoid=36.8%, android-anthropoid=38.9%, android-flat=41.2%, anthropoid (pure)=28.7%, anthropoid-gynecoid=9.3%, anthropoid-android=13.0%, flat (pure)=23.7%, flat-gynecoid=22.0%, flat-android=56.0%. These probability statistics emphasize the importance of accurate assessment of the pelvic dimensions and architecture by pelvimetry.
Manual pelvimetry done properly is about as accurate as radiographic pelvimetry in predicting obstetric outcome; moreover, manual pelvimetry has no longterm effects, while X-rays are associated with an increased incidence of leukemia and other childhood malignancies. Radiographic pelvimetry is usually contraindicated, especially during the first trimester, with exceptions such as contemplation of vaginal delivery with breech presentation. It should be noted that in this case, as in most, the indication for radiographic pelvimetry appears near the end of the term; some type of pelvimetry would have already been performed during the initial examination at the inception of the pregnancy. Magnetic resonance imaging (MRI) is currently touted as a viable alternative pelvimetry method, with the advantage that fetal cerebral energy metabolism may be estimated and the detection of fetal hypoxia may be possible. However, MRI imaging availability is sparse and expensive, as it is an emerging imaging modality, and is not readily available, especially in depressed and rural areas, and more especially in less developed and third-world countries.
The capacity of the bony pelvis can be estimated accurately enough for practical purposes by careful clinical examination. Long experience is necessary. Since X-ray films and mechanical methods are no substitute for the careful assessment of the bones of the pelvic girdle, clinical examination may have to be repeated during the course of pregnancy and even during labor if progress is unsatisfactory. It may be best to delay definitive typing and mensuration of the pelvis until shortly before term since patient discomfort may be less when the fetus has achieved maximal size.
In modern obstetrics, X-ray pelvimetry is seldom needed. Occasionally it is used in cases of trauma, fracture and deformity, although even in these cases ultrasonography may yield the necessary information. X-ray pelvimetry usage must be tempered by an appreciation of the genetic radiation hazards involved plus the knowledge that it does not evaluate all of the numerous factors determining the outcome of any given labor. X-ray films thus are not needed in most cases. As with most aspects of obstetrics, the management of possible bony pelvic dystocia demands clinical judgement of the highest order in the best interests of mother and infant.
Coel, U.S. Pat. No. 2,495,568 provides a clinical model of the female pelvis region. While he teaches that it is desirable to use separate castings of the two pelvic halves, he does not teach use of posterior and anterior pelvic segments of different anatomical types. Indeed, his sections are "substantially mirror images of each other." Moreover, it is not even clear that his model specifically includes the bony pelvis. Coel certainly does not discuss the bony pelvis and its morphology, or suggest the use of his model as a pelvimetry teaching aid.
Jacobs, U.S. Pat. No. 2,127,774 describes a manikin for teaching obstetrics which includes major elements of the pelvic bone construction. There is no provision for interchangeably combining posterior and anterior pelvic segments of the various anatomical types.
Kittner, U.S. Pat. No. 3,213,550 discloses an anatomical model which can receive either a male or a female pelvic insert.